Optical semiconductor device and method for fabricating the same

ABSTRACT

An optical semiconductor device includes: a first conductivity type first semiconductor region; a first conductivity type second semiconductor region formed on the first semiconductor region; a second conductivity type third semiconductor region formed on the second semiconductor region; a photodetector section formed of the second semiconductor region and the third semiconductor region; a micro mirror formed of a trench formed selectively in a region of the first semiconductor region and the second semiconductor region except the photodetector section; and a semiconductor laser element held on the bottom face of the trench. A first conductivity type buried layer of which impurity concentration is higher than those of the first semiconductor region and the second semiconductor region is selectively formed between the first semiconductor region and the second semiconductor region in the photodetector section.

CROSS REFERENCE TO RELATED APPLICATIONS

This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2003-417792 filed in Japan on Dec. 16,2003, the entire contents of which are hereby incorporated by reference.

BACKGROUND ART

The present invention relates to an optical semiconductor device inwhich a photodetector and a semiconductor laser element are formed on asingle substrate and a method for fabricating the same.

A light emitting element and a photodetector are elements for mutualconversion between an optical signal and an electric signal, and areemployed in various kinds of art field. In the field of optical diskssuch as CDs (Compact Discs) and DVDs (Digital Versatile Disc), they aremain devices in optical pickups for reading/writing signals recorded onan optical disks.

In recent years, in accordance with demand for high performance and highintegration, a photodiode serving as a photodetector and variouselectronic elements such as a bipolar transistor, a resister, acapacitance, are formed on a single substrate to compose a so-calledopto-electronic integrated circuit (OEIC) device. For further sizereduction and higher integration, OEIC devices are widely used in whicha semiconductor laser element as a light emitting element and a micromirror for changing a light path of the laser beam output form thesemiconductor laser element are mounted. The OEIC devices of this kindare generally formed by a bipolar transistor fabricating method. Inaddition, the OEIC devices are required to include both a photodetectorhaving high photosensitivity, high-speed operability and low noisecharacteristics and high-speed, highly accurate bipolar transistor.

A conventional optical semiconductor device will be described below withreference to the drawings.

FIG. 10 shows schematically a sectional construction of an opticalsemiconductor device, that is, an OEIC device according to theconventional example. As shown in FIG. 10, a N-type epitaxial layer 102is formed on a principal surface of a semiconductor substrate 101 madeof P-type low impurity concentration silicon.

In the semiconductor substrate 101 and the N-type epitaxial layer 102, atransistor section 200 composed of a NPN bipolar transistor, aphotodetector section 220 composed of a PIN photodiode and a lightemitting element section 240 including a semiconductor laser chip 125are formed to compose the OEIC device.

The transistor section 200, which is a two-layer polysilicon selfaligned type NPN transistor, is composed of: a high concentration N-typeemitter region 106; a P-type base region 107 formed below the emitterregion 106; a collector region 108 made of the N-type epitaxial layer102 and formed below the base region 7; a high concentration N-typecollector buried region 109 formed below the collector region 108; anemitter electrode 110 formed above the emitter region 106; a baseelectrode 111 connected electrically to the peripheral portion of thebase region 107; and a collector electrode 112 formed above thecollector buried region 109 and connected electrically to the endportion of the collector buried region 109.

The light receiving section 220 is composed of: a cathode layer 115 madeof the N-type epitaxial layer 102; a high concentration N-type cathodesurface layer 116 formed on the cathode layer 115; a high concentrationN-type cathode contact layer 117 formed around the cathode surface layer116; and a cathode electrode 118 formed above the cathode contact layer117.

In each of the transistor section 200 and the photodetector section 220,an isolation oxide film 113 for electrically isolating the elements isformed by local thermal oxidation, that is, so-called LOCOS. A highconcentration P⁺-type isolation layer 114 is formed below the isolationoxide film 113.

In the photodetector section 220, the P⁺-type isolation layer 114located in the peripheral portion of the photodetector section 220 inthe semiconductor substrate 101 functions as a part of an anode and isconnected electrically to an anode electrode 120 with the interventionof a high concentration P-type anode contact layer 119 formed on theP⁺-type isolation layer 114. A portion of the low concentration P-typesemiconductor substrate 101 located below the cathode layer 115 servesas an anode region, and is taken outside as a current from the anodeelectrode 120 through the P⁺-type isolation layer 114 and the anodecontact layer 119. On the cathode surface layer 116 serving as a lightreceiving face, an anti-reflection film 121 is provided for reducingreflection of incident light 122 on the cathode surface layer 116.

In the light emitting element section 240, a micro mirror region 123 isformed which is formed of a trench formed by digging the N-typeepitaxial layer 102 and the upper part of the semiconductor substrate101 by anisotropic etching. On the bottom face of the trench, asemiconductor laser chip 125 is fixed with the intervention of a laserlower electrode 128, a laser wire 127 and a protection film 126. Thelaser wire 127 is lead outside the trench along the wall face from thebottom face of the trench. The protection film 126 is formed so as tocover each upper face of the transistor section 200 and thephotodetector section 220.

As shown in FIG. 10, laser light emitted from a side facet of thesemiconductor laser chip 125 is reflected on the surface of the micromirror region 123 to be output in a direction approximatelyperpendicular to the principal surface of the semiconductor substrate110.

The operation of the thus composed OEIC device will be described below.

Application of a current over a threshold value to the semiconductorlaser chip 125 causes induced emission and oscillation, so that coherentlaser light 129 is output in a direction parallel to the principalsurface of the semiconductor substrate 101. In the case where the micromirror region 123 forms an angle at 45 degrees with respect to thesubstrate surface, the emitted laser light 129 is reflected on thesurface of the micro mirror region 123 to rise in a directionperpendicular to the substrate surface. The reflected laser light 129 isirradiated on, for example, an optical disk or the like and a part ofthe thus reflected light becomes incident light 122 to enter in thephotodetector section 220.

The incident light 122 that enters in the photodetector section 220 isabsorbed in the semiconductor substrate 101 serving as the anode and thecathode layer 115 to generate electron hole pairs. When reverse biasvoltage is applied to the photodetector section 220 at that time, adepletion layer is extended toward the semiconductor substrate 101 whereimpurity concentration is low. The electron hole pairs generated in theextended depletion layer and the vicinity thereof diffuse and driftseparately so that the electrons and the holes reach the cathode contactlayer 117 and the anode contact layer 119, respectively, therebygenerating a photocurrent. Upon receiving the thus generatedphotocurrent, an electronic circuit composed of a NPN transistor, aresistor, a capacitor and the like performs predetermined amplificationand signal processing to output the photocurrent as a recording orreplay signal of an optical disk.

As described above, in recent years, in optical semiconductor deviceshaving photodetector for optical pickup used in CDs and DVDs, highphotosensitivity, high-speed operability and downsizing are stronglydemanded in association with high-speed driving of optical disks andincreasing density of recorded signals.

In the aforementioned conventional optical semiconductor device,however, the photocurrent generated from incident light is divided tothe diffusion current component and the drift current component asdescribed above, wherein the diffusion current component is dominantdiffusion that the minority carriers move up to the end portion of thedepletion layer. For this reason, the response speed of the diffusioncurrent component is lower than the drift current component drifting bythe electric field in the depletion layer, which is a factor ofdeterioration the frequency characteristic of the photodetector section220 made of a photodiode.

Especially, infrared light used in CDs, which has a small absorptioncoefficient to silicon, reaches deep inside of the semiconductorsubstrate 101 and carriers generated at the deep part contributes to thecurrent, which restrict high-speed operation. In this connection, it isimpossible to form the photodetector section 220 and the light emittingelement section 240 integrally on a single semiconductor substrate 101for exhibiting high photosensitivity and high-speed operation.

SUMMARY OF THE INVENTION

The present invention has its object of forming on a single substrate aphotodetector section having high photosensitivity and high-speedoperability and a light emitting element section in which asemiconductor laser chip is mounted by solving the above conventionalproblems.

The present inventor fabricated an optical semiconductor device havingthe construction shown in FIG. 11 for achieving the above object. Theoptical semiconductor device having this construction will be describedbelow as a reference example.

FIG. 11 shows a schematic sectional construction of the opticalsemiconductor device according the reference example of the presentinvention. In FIG. 11, the same reference numerals are assigned to thesame members as those shown in FIG. 10 and the description thereof isomitted.

As shown in FIG. 11, in the optical semiconductor device according tothe reference example, a P-type anode buried layer 101 a of whichimpurity concentration is three-or-more-digit larger, namely 10³ timeslarger than that of a semiconductor substrate 101 is formed entirely ina region under a P⁺-type isolation layer 114 in the semiconductorsubstrate 101 made of low impurity concentration P-type silicon. AP⁻-type epitaxial layer 103 of which impurity concentration isapproximately the same as that of the semiconductor substrate 101 isformed on the anode buried layer 101 a.

Accordingly, a transistor section 200 and a photodetector section 220are formed of the P⁻-type epitaxial layer 103 and the N-type epitaxiallayer 102 grown thereon, and a trench of a light emitting elementsection 240 is formed through the N-type epitaxial layer 102, theP⁻-type epitaxial layer 103 and the anode buried layer 101 a.

As described above, when the difference in impurity concentrationbetween the semiconductor substrate 101 and the anode buried layer 101 ais set to be three digits or more, the carriers generated by lightabsorbed in the semiconductor substrate 100 are inhibited from diffusionby a potential barrier caused due to concentration gradient and arere-coupled, with a result of no contribution to the current(photocurrent). If the thickness of the P⁻-type epitaxial layer 103 isset so that the end part of the depletion layer reaches the anode buriedlayer 101 a, the drift current is dominant in the photocurrent,resulting in high-speed operation.

Further, though the generated holes moves from the P⁻-type epitaxiallayer 103 to the anode buried layer 101 a and to the anode contact layer119 through the P⁺-type isolation layer 114, the provision of the anodeburied layer 101 a, which is a high concentration layer, enableshigh-speed response with smaller series resistance than that in the casewith no anode buried layer 101 a provided.

The. N-type epitaxial layer 102, which serves as the collector region108 of the NPN bipolar transistor composing the transistor section 200,must not have low concentration, and accordingly, the cathode layer 115must not be depleted. For this reason, the concentration of the cathodesurface layer 116 is set higher than that of the cathode layer 115 so asto enhance the photosensitivity and the frequency characteristic tolight of short wavelength almost of which is absorbed in the vicinity ofthe surface of the N-type epitaxial layer 102 for attain high efficiencyof photoelectric conversion by utilizing the concentration gradient.Concentration difference necessary for attain the high efficiency of thephotoelectric conversion is se to be three or more digits.

With the above construction, the optical semiconductor device accordingto the reference example of the present invention sufficiently ensuresan effective region of the depletion layer by utilizing the impurityconcentration difference between the low impurity concentration portion(P⁻-type epitaxial layer 103) contributing to the photosensitivity ofthe photodetector section 220 and the surface (cathode surface layer116) of the photodetector section 220. As a result, the frequencycharacteristic and the photosensitivity even to light having a shortoptical absorption length can be enhanced and the capacitance can bereduced without deterioration of the operation characteristic of thetransistor section 200.

In the optical semiconductor device according to the reference example,the light path is changed in the direction perpendicular to thesubstrate surface by reflecting the emitted light 129 on the micromirror region 123 provided as the trench in the light emitting elementsection 240. Therefore, the surface of the micro mirror region 123 isdemanded to be flat with high precision.

Anisotropic wet etching is used for forming the trench to be the micromirror region 123 in general. Potassium hydroxide (KOH), for example, isused for an etching solution for the anisotropic wet etching. However,due to a large difference in impurity concentration between the P⁺-typeanode buried layer 101 a and the P⁻-type epitaxial layer 103, the etchrate becomes different between the anode buried layer 101 a and theP⁻-type epitaxial layer 103. The difference in etch rate causes a lineand further causes a pit, which is developed from an impurity in theanode buried layer 101 a due to crystal defect, in the micro mirrorregion 123 formed of the wall face of the trench. The emitted light 129may be scattered at the line or the pit and the rising angle of thereflected light may vary.

Taking account of the above problems, the present inventor has conductedvarious researches on the reference example to find that in order toform on a single substrate a transistor section capable of high-speedoperation, a photodetector section having high photosensitivity andhigh-speed operability and a light emitting element section in which asemiconductor laser chip is mounted, in an optical semiconductor device,the photodetector section 220 in which the anode buried layer 101 a isprovided is formed selectively only under the photodetector section 220so that the anode buried layer 101 a is not exposed at the micro mirrorregion 123 formed of the wall face of the trench of the light emittingelement section 240.

Specifically, an optical semiconductor device according to the presentinvention includes: a first conductivity type first semiconductorregion; a first conductivity type second semiconductor region formed onthe first semiconductor region; a second conductivity type thirdsemiconductor region formed on the second semiconductor region; aphotodetector section formed of the second semiconductor region and thethird semiconductor region; a micro mirror formed of a trench formedselectively in a region of the first semiconductor region and the secondsemiconductor region except the photodetector section; and asemiconductor laser element held on a bottom face of the trench, whereina first conductivity buried layer of which impurity concentration ishigher than those of the first semiconductor region and the secondsemiconductor region is formed between the first semiconductor regionand the second semiconductor region in the photodetector section.

In the optical semiconductor device according to the present invention,an effective region of the depletion layer is ensured sufficiently byutilizing the difference in impurity concentration between the secondsemiconductor region that contributes to the photosensitivity of thephotodetector section and the third semiconductor region, with a resultthat the frequency characteristic and the photosensitivity for lighthaving short optical absorption wave are enhanced and the capacitance isreduced. Further, with no buried layer exposed at the micro mirrorformed of the trench, a line and a pit caused by crystal defect, whichare generated due to difference in etch rate generated by difference inimpurity concentration between the buried layer and the secondsemiconductor region are prevented, with a result that the micro mirrorexcellent in flatness can be obtained. Hence, a photodetector havinghigh-speed operability and high photosensitivity and a semiconductorlaser element can be formed on a single substrate without deteriorationof the optical characteristics.

In the optical semiconductor device of the present invention, it ispreferable to form the second semiconductor region by epitaxial growth.

In the optical semiconductor device of the present invention, it ispreferable to form the third semiconductor region by epitaxial growth.

The optical semiconductor device of the present invention preferablyincludes a transistor formed in a region of the second semiconductorregion and the third semiconductor region except the photodetectorsection and the trench.

A first optical semiconductor device fabrication method according to thepresent invention includes the steps of: forming selectively a firstconductivity type buried layer of which impurity concentration is higherthan that of a first conductivity type first semiconductor region by ionimplantation to a photodetector section formation portion of the firstsemiconductor region; forming by epitaxial growth a first conductivitytype second semiconductor region of which impurity concentration islower than that of the buried layer on the first semiconductor region inwhich the buried layer is formed; forming a second conductivity typethird semiconductor region in the upper part of the second semiconductorregion; forming a photodetector section made of the second semiconductorregion and the third semiconductor region in the photodetector sectionformation portion of the second semiconductor region and the thirdsemiconductor region; forming, by forming a trench by performingselective anisotropic etching on a region of the first semiconductorregion and the second semiconductor region except the photodetectorsection, a micro mirror formed of a wall face of the trench; and bondinga semiconductor laser element, which is prepared beforehand in a form ofa chip, onto a bottom face of the thus formed trench.

In the first optical semiconductor device fabrication method, the firstconductivity type second semiconductor region of which impurityconcentration is lower than that of the buried layer is formed on thefirst semiconductor region in which the buried layer is formed in thephotodetector section formation portion selectively, and the secondconductivity type third semiconductor region is formed on the secondsemiconductor region. Then, the trench is formed by selectiveanisotropic etching to the region except the photodetector section.Accordingly, the buried layer of which impurity concentration is higherthan that of the first semiconductor region is not exposed at the wallface of the trench forming the micro mirror. Hence, the opticalsemiconductor device according to the present invention can be realized.

A second optical semiconductor device fabrication method according tothe present invention includes the steps of: forming selectively a firstconductivity type buried layer of which impurity concentration is higherthan that of a first conductivity type first semiconductor region by ionimplantation to a photodetector section formation portion of the firstsemiconductor region; forming by epitaxial growth a first conductivitytype second semiconductor region of which impurity concentration islower than that of the buried layer on the first semiconductor region inwhich the buried layer is formed; forming a second conductivity typethird semiconductor region on the second semiconductor region byepitaxial growth; forming a photodetector section made of the secondsemiconductor region and the third semiconductor region in thephotodetector section formation portion of the second semiconductorregion and the third semiconductor region; forming, by forming a trenchby performing selective anisotropic etching on a region of the firstsemiconductor region and the second semiconductor region except thephotodetector section, a micro mirror formed of a wall face of thetrench; and bonding a semiconductor laser element, which is preparedbeforehand in a form of a chip, onto a bottom face of the thus formedtrench.

In this way, in the second optical semiconductor device fabricationmethod, the second conductivity type third semiconductor region in thefirst fabrication method is epitaxial grown.

A third optical semiconductor device fabrication method according to thepresent invention includes the steps of: forming a first conductivitytype second semiconductor region on a first conductivity type firstsemiconductor region by epitaxial growth; forming selectively a firstconductivity type buried layer of which impurity concentration is higherthan that of the first semiconductor region by ion implantation to aboundary portion between the first semiconductor region and the secondsemiconductor region and a photodetector section formation portion inthe vicinity of the boundary portion; forming a second conductivity typethird semiconductor region in an upper part of the second semiconductorregion; forming a photodetector section made of the second semiconductorregion and the third semiconductor region in a photodetector sectionformation portion of the second semiconductor region and the thirdsemiconductor region; forming, by forming a trench by performingselective anisotropic etching on a region of the first semiconductorregion and the second semiconductor region except the photodetectorsection, a micro mirror formed of a wall face of the trench; and bondinga semiconductor laser element, which is prepared beforehand in a form ofa chip, onto a bottom face of the thus formed trench.

In this way, in the third optical semiconductor device fabricationmethod, the second semiconductor region is epitaxially grown on thefirst semiconductor region, and then, the first conductivity type buriedlayer of which impurity concentration is higher than that of the firstsemiconductor region is formed selectively in the boundary portionbetween the first semiconductor region and the thus formed secondsemiconductor region and the photodetector section formation portion inthe vicinity of the boundary portion.

A fourth optical semiconductor device fabrication method according tothe present invention includes the steps of: forming a firstconductivity type second semiconductor region on a first conductivitytype first semiconductor region by epitaxial growth; forming selectivelya first conductivity type buried layer of which impurity concentrationis higher than that of the first semiconductor region by ionimplantation to a boundary portion between the first semiconductorregion and the second semiconductor region and a photodetector sectionformation portion in the vicinity of the boundary portion; forming asecond conductivity type third semiconductor region on the secondsemiconductor region by epitaxial growth; forming a photodetectorsection made of the second semiconductor region and the thirdsemiconductor region in a photodetector section formation portion of thesecond semiconductor region and the third semiconductor region; forming,by forming a trench by performing selective anisotropic etching on aregion of the first semiconductor region and the second semiconductorregion except the photodetector section, a micro mirror formed of a wallface of the trench; and bonding a semiconductor laser element, which isprepared beforehand in a form of a chip, onto a bottom face of the thusformed trench.

In this way, in the fourth optical semiconductor device fabricationmethod, the second conductivity type third semiconductor region in thethird fabrication method is epitaxialy grown.

It is preferable that the first to fourth optical semiconductor devicefabrication methods further includes the step of: forming selectively atransistor in a region of the second semiconductor region and the thirdsemiconductor region except the photodetector section and the trench.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic construction in section of an opticalsemiconductor device according to the first embodiment of the presentinvention.

FIG. 2A through to FIG. 2F are sections showing the step sequence of afirst fabrication method of the optical semiconductor device accordingto the first embodiment of the present invention.

FIG. 3A through to FIG. 3D are sections showing the step sequence of asecond fabrication method of the optical semiconductor device accordingto the first embodiment of the present invention.

FIG. 4 shows a schematic construction in section of an opticalsemiconductor device according to the second embodiment of the presentinvention.

FIG. 5A through to FIG. 5G are sections showing the step sequence of afirst fabrication method of the optical semiconductor device accordingto the second embodiment of the present invention.

FIG. 6A through to FIG. 6E are sections showing the step sequence of asecond fabrication method of the optical semiconductor device accordingto the second embodiment of the present invention.

FIG. 7 shows a schematic construction in section of an opticalsemiconductor device according to the third embodiment of the presentinvention.

FIG. 8A through to FIG. 8G are sections showing the step sequence of afirst fabrication method of the optical semiconductor device accordingto the third embodiment of the present invention.

FIG. 9A through to FIG. 9E are sections showing the step sequence of asecond fabrication method of the optical semiconductor device accordingto the third embodiment of the present invention.

FIG. 10 shows a schematic construction in section of a conventionaloptical semiconductor device.

FIG. 11 shows a schematic construction in section of an opticalsemiconductor device according to the reference example of the presentinvention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION FIRST EMBODIMENT

The first embodiment of the present invention will be described belowwith reference to the drawings.

FIG. 1 shows a schematic construction in section of an opticalsemiconductor device (OEIC device) according to the first embodiment ofthe present invention. As shown in FIG. 1, a low concentrationP⁻-epitaxial layer 2 having, for example, a thickness of about 10 μm andan impurity concentration of about 1×10¹⁴ cm⁻³ is formed on theprincipal surface of a semiconductor substrate 1 made of P-type lowimpurity concentration silicon (Si).

In the semiconductor substrate 1 and the P⁻-type epitaxial layer 2, aphotodetector section 220 made of a PIN photodiode and a light emittingelement section 240 including a semiconductor laser chip 25 are formedto compose the OEIC device.

The photodetector section 220 includes: a N-type cathode surface layer16 having a thickness of, for example, 0.1 μm and formed in the upperpart of the P⁻-type epitaxial layer 2; a high concentration P⁺-typeanode buried layer 1 a between the semiconductor substrate 1 and theP⁻-type epitaxial layer 2 and having, for example, a thickness of about4 μm and an impurity concentration of at least 1×10¹⁷ cm⁻³; a cathodecontact layer 17 formed in the peripheral portion of the cathode surfacelayer 16; and a cathode electrode 18 formed on the cathode contact layer17. The photodetector section 220 further includes: double isolationoxide films 13 formed in the peripheral portion of the cathode contactlayer 17 with a space left from each other by LOCOS or the like; ananode contact layer 19 formed between adjacent isolation oxide films 13;a high concentration P⁺-type isolation layer 14 formed under the anodecontact layer 19 in the P⁻-type epitaxial layer 2 and functioning as apart of an anode; and an anode electrode 20 formed on the anode contactlayer 19.

In the light emitting element section 240, a micro mirror region 23 isprovided which is formed of a trench formed by digging the P⁻-typeepitaxial layer 2 and the upper part of the semiconductor substrate 1 byanisotropic etching. A semiconductor laser chip 25 is fixed at thebottom of the trench, with a laser lower electrode 28, a laser wire 27and a protection film 26 intervened. The laser 27 wire is lead outsidethe trench along the bottom face and the wall face of the trench. Theprotection film 26, which is made of silicon oxide or silicon nitride,is formed so as to cover also the upper face of the photodetectorsection 220.

The operation of the thus constructed optical semiconductor device willbe described below.

Application of a current over a threshold value to the semiconductorlaser chip 25 causes induced emission and oscillation, so that coherentlaser light 29 is output in a direction parallel to the principalsurface of the semiconductor substrate 1. In the case where the micromirror region 23 forms an angle at 45 degrees with respect to thesubstrate surface, the emitted laser light 29 is reflected on thesurface of the micro mirror region 23 to rise in a directionperpendicular to the substrate surface. The reflected laser light 29 isirradiated on, for example, an optical disk or the like and a part ofthe thus reflected light becomes incident light 22 to enter in thephotodetector section 220.

The incident light 22 that enters in the photodetector section 220 isabsorbed in the cathode surface layer 16 and the P⁻-type epitaxial layer2, to generate electron hole pairs. When reverse bias voltage is appliedto the photodetector section 220 at this time, a depletion layer isextended toward the P⁻-type epitaxial layer 2 where impurityconcentration is low. The electron hole pairs generated in the extendeddepletion layer and the vicinity thereof diffuse and drift separately sothat the electrons and the holes reach the cathode contact layer 17 andthe anode contact layer 19, respectively, thereby generating aphotocurrent. Namely, an optical signal is converted and output as anelectric signal.

Referring to the features of the first embodiment, the P⁺-type anodeburied layer 1 a is selectively provided in the semiconductor substrate1 and the P⁻-type epitaxial layer 2 only in the photodetector section220. Accordingly, when a difference in impurity concentration betweenthe semiconductor substrate 1 and the P⁺-type anode buried layer 1 a isset to be three digits or more, the carriers generated in thesemiconductor substrate 1 are inhibited from diffusion by a potentialbarrier generated due to concentration gradient and are re-coupled, witha result of no contribution to the photocurrent. Hence, the driftcurrent becomes dominant, thereby enabling high-speed operation.

At the same time, the low concentration P⁻-type epitaxial layer 2 havingthe thickness of about 10 μm and the impurity concentration of about1×10¹⁴ cm⁻³ is completely depleted and has 80% or higher opticalabsorption rate to red and infrared light. As a result, a photodiodehaving high-speed operability and high photosensitivity to light havingwavelengths from red to infrared is realized.

In addition, the P⁺-type anode buried layer 1 a is not provided in thelight emitting element section 240, and accordingly, no difference inimpurity concentration is caused between the semiconductor substrate 1and the P⁻-type epitaxial layer 2, both of which have low concentration.In this connection, no difference in etch rate, which is caused due todifference in impurity concentration, is caused in the process offorming the micro mirror region 23 by anisotropic etching using analkaline solution, thereby preventing a line and a pit, which isdeveloped due to crystal defect from an impurity in the highconcentration P⁺-type anode buried layer 1 a, in the mirror faceportion. Thus, the micro mirror region 23, which is excellent inflatness, can be formed.

FIRST FABRICATION METHOD IN FIRST EMBODIMENT

A first fabrication method of the optical semiconductor deviceconstructed as above will be described below with reference to thedrawings. FIG. 2A through FIG. 2F are sections showing the sequence ofthe first fabrication method of the optical semiconductor deviceaccording to the first embodiment of the present invention.

First, as shown in FIG. 2A, a protection oxide film 35 made of siliconoxide is formed on the principal surface of a semiconductor substrate 1made of P⁻-type silicon, and a resist pattern 36 as a mask for coveringthe protection oxide film 35 in a light emitting element section 240 isformed by a lithography method. Then, a P⁺-type anode buried layer 1 ais formed selectively in a region of the semiconductor substrate 1except the light emitting element section 240 by ion implantation of,for example, boron (B) ion as a P-type impurity to the semiconductorsubstrate 1 through the protection oxide film 35, using the resistpattern 36 as a mask. Herein, the dose amount of the boron ion is set tobe 5×10^(14 cm) ⁻² and the acceleration energy is set to be 30 keV, forexample.

Subsequently, as shown in FIG. 2B, the resist pattern 36 is removed byashing or the like, and thermal treatment, for example, at a temperatureof about 1100° C. for about 30 minutes is performed on the semiconductorsubstrate 1 in which the boron ion is implanted to activate theimplanted boron ion. Then, the protection oxide film 35 is removed byetching using an etching solution of which main component ishydrofluoric acid or an etching gas of which main component isfluorocarbon, and then, a P⁻-type anode epitaxial layer 2 of about 10 μmin thickness is formed by chemical vapor deposition (CVD) on theprincipal surface of the semiconductor substrate 1 in which the P⁺-typeanode buried layer 1 a is formed.

Next, as shown in FIG. 2C, a photodiode element and an isolation oxidefilm 13 are formed in the upper part of the P⁻-type epitaxial layer 2respectively in the photodetector section 220 and the light emittingelement section 240 by an ordinary photodiode formation step.

Referring to the photodiode, boron ion as a P-type impurity is implantedselectively in the upper part of the P⁻-type epitaxial layer 2 in theperipheral portion of the photodetector section 220 to have aconcentration of 1×10¹⁷ cm⁻³ to 1×10¹⁸ cm⁻³, to form a P⁺-type isolationlayer 14.

Subsequently, an isolation oxide film 13 made by LOCOS is formed in theupper part of the P⁻-type epitaxial layer 2 entirely in the lightemitting element section 240 and other isolation oxide films 13 areformed double in the upper part of the P⁻-type epitaxial layer 2 on theP⁺-type isolation layer 4 in the light emitting element section 220,with an interval left therebetween.

Next, a P-type polysilicon layer is selectively formed between theisolation oxide films 13 in the upper part of the P⁻-type epitaxiallayer 2 to form a P-type anode contact layer 19 between the isolationoxide films 13 in the upper part of the P⁻-type epitaxial layer 2 bysolid phase diffusion from the thus formed polysilicon layer.

Subsequently, a N-type polysilicon layer is formed on the P⁻-typeepitaxial layer 2 to form a N-type cathode contact layer 17 by solidphase diffusion form the thus formed polysilicon layer. Then, a N⁺-typecathode surface layer 16 is formed by ion implantation with arsenic (As)or phosphorous (P) ion to a region surrounded by the inner isolationoxide film 13 in the upper part of the P⁻-type epitaxial layer 2.

Next, a cathode electrode 18 and an anode electrode 20 are formed on theN-type polysilicon layer and the P-type polysilicon layer, respectively.It is noted that a lamination structure of a metal layer of which maincomponent is titanium (Ti) and a metal layer of which main component isaluminum (Al) can be employed as a material of each electrode 18, 20.Then, an anti-reflection film 21 made of, for example, silicon oxide isformed on at least the cathode surface layer 16, and a protection film26 is formed on the anti-reflection film 21 by CVD.

Subsequently, as shown in FIG. 2D, a trench formation region in theisolation oxide film 13 formed in the light emitting element section 240is selectively etched and removed, whereby a opening pattern for openingthe trench formation region is formed in a region of the isolation oxidefilm 13 in the light emitting element section 240. Then, a trench isformed by anisotropic wet etching on the P⁻-type epitaxial layer 2 andP⁻-type semiconductor substrate 1 with an alkaline solution such as anaqueous solution of potassium hydroxide (KOH), using as a mask theisolation oxide film 13 of the opening pattern which remains in thelight emitting element section 240, and then, a micro mirror region 23is formed at the wall face of the thus formed trench. It is noted that a(111) plane in orientation of silicon has the latest silicon etch rateof alkaline solutions, and therefore, the (111) plane becomes as themirror face of the micro mirror region 23. Wherein, in the case wherethe orientation of the principal surface of the semiconductor substrate1 made of silicon is in a (100) plane, the angle between the wall faceand the bottom face of the trench is 54.7 degrees, as conventionallyknown. Accordingly, when an inclined substrate that inclines 9.7 degreesfrom the (100) plane as the orientation of the principal surface of thesemiconductor substrate 1 is used, namely, an offset substrate is used,the angle between the wall face and the bottom face of the trench is 45degrees. Thus, the micro mirror region 23 can form the angle of 45degrees with respect to the principal surface of the semiconductorsubstrate 1, and accordingly, a light path of the laser light emitted inparallel to the substrate surface can be changed in a directionperpendicular to the substrate surface.

Next, as shown in FIG. 2E, a protection film 26 is formed by CVD so asto cover the exposed face of the light emitting element region 240 inwhich the micro mirror region 23 of the trench is formed. Then, a laserwire 27 of which main component is, for example, gold (Au) is formed bya vapor deposition method or a sputtering method so as to extend fromthe bottom face to the outside of the trench. Thereafter, a laser lowerelectrode 28 is selectively formed on the laser wire 27 on the bottomface of the trench by a deposition method, a sputtering method, afield-effect plating method or the like.

Subsequently, as shown in FIG. 2F, a semiconductor laser chip 25 isbonded on the laser lower electrode 28, thereby obtaining the opticalsemiconductor device according to the first embodiment.

SECOND FABRICATION METHOD IN FIRST EMBODIMENT

A second fabrication method of the optical semiconductor deviceaccording to the first embodiment will be described below with referenceto FIG. 3A through FIG. 3D. In the second fabrication method, a P⁺-typeanode buried layer 1 a is formed after a P⁻-type epitaxial layer 2 isformed on the principal surface of a semiconductor substrate 1.

First, as shown in FIG. 3A, the P⁻-type epitaxial layer 2 of about 10 μmin thickness is formed on the principal surface of the semiconductorsubstrate 1 by CVD.

Next, as shown in FIG. 3B, a protection oxide film 35 made of siliconoxide is formed on the P⁻-type epitaxial layer 2, and a resist pattern36 as a mask for covering the protection oxide film 35 in a lightemitting element section 240 is formed by a lithography method. Then, aP⁺-type anode buried layer 1 a is formed selectively in a region betweenthe semiconductor substrate 1 and the P⁻-type epitaxial layer 2 exceptthe light emitting element section 240 by ion implantation of, forexample, boron (B) ion as a P-type impurity to the semiconductorsubstrate 1 through the protection oxide film 35 and the P⁻-typeepitaxial layer 2, using the resist pattern 36 as a mask. Herein, thedose amount of the boron ion is set to be 5×10¹⁴ cm⁻² and theacceleration energy is set to be 2 MeV, for example. Thereafter, theresist pattern 36 is removed by ashing or the like, and thermaltreatment, for example, at a temperature of about 1100° C. for about 30minutes is performed on the semiconductor substrate 1 in which the boronion is implanted to activate the boron ions.

Subsequently, as shown in FIG. 3C, after the protection oxide film 35 isremoved, an isolation oxide film 13 is formed in the P⁻-type epitaxiallayer 2 in the photodetector section 220 selectively and an isolationoxide film 13 is formed in the P⁻-type epitaxial layer 2 in the lightemitting element section 240 entirely.

Next, as shown in FIG. 3D, a photodiode is formed in the photodetectorsection 220, and a micro mirror region 23 of a trench is formed in thelight emitting element section 240 by etching using the isolation oxidefilm 13 remaining in the light emitting element section 240 as a mask.Thereafter, likewise the first fabrication method, a semiconductor laserchip 25 is bonded on the bottom face of the trench, thereby obtainingthe optical semiconductor device according to the first embodiment.

SECOND EMBODIMENT

The second embodiment of the present invention will be described belowwith reference to the drawings.

FIG. 4 shows a schematic construction in section of an opticalsemiconductor device (OEIC device) according to the second embodiment ofthe present invention. In FIG. 4, the same reference numerals areassigned to the same members as those in FIG. 1 and the descriptionthereof is omitted. In the optical semiconductor device according to thesecond embodiment, the N-type epitaxial layer 3 is provided on theP⁻-type epitaxial layer 2.

Accordingly, a cathode in the photodiode composing the photodetectorsection 220 is composed of the cathode layer 25 of the N-type epitaxiallayer 3 and the cathode surface layer 16 formed in the upper part of thecathode layer 15. Herein, the impurity concentration of the cathodesurface layer 16 is set to be three-digit larger, that is, 10³ timeslarger than the impurity concentration of the cathode layer 3 so as toenhance the efficiency of photoelectric conversion, with a result ofenhancement of the photosensitivity and the frequency characteristic toincident light 22 absorbed in the vicinity of the surface portion of thecathode layer 15.

Similar to the first embodiment, the P⁺-type anode buried layer 1 a isnot provided in the light emitting element section 240, and accordingly,no difference in impurity concentration is caused between thesemiconductor substrate 1 and the P⁻-type epitaxial layer 2, both ofwhich have low concentration. In this connection, no difference in etchrate, which is caused due to difference in impurity concentration, iscaused in the process of forming the micro mirror region 23 byanisotropic etching using an alkaline solution, thereby preventing aline and a pit, which is developed due to crystal defect from animpurity in the high concentration P⁺-type anode buried layer 1 a, inthe mirror face. Thus, the micro mirror region 23, which is excellent inflatness, can be formed.

FIRST FABRICATION METHOD IN SECOND EMBODIMENT

A first fabrication method of the optical semiconductor device asconstructed as above will be described below with reference to thedrawings. FIG. 5A through FIG. 5G are sections showing the sequence ofthe first fabrication method of the optical semiconductor deviceaccording to the second embodiment of the present invention.

First, as shown in FIG. 5A, likewise the first fabrication method in thefirst embodiment, a protection oxide film 35 made of silicon oxide isformed on the principal surface of a semiconductor substrate 1 made ofP⁻-type silicon, and a resist pattern 36 as a mask for covering theprotection oxide film 35 in a light emitting element section 240 isformed. Then, a P⁺-type anode buried layer 1 a is formed selectively ina region of the semiconductor substrate 1 except the light emittingelement section 240 by ion implantation of, for example, boron (B) ionas a P-type impurity to the semiconductor substrate 1 through theprotection oxide film 35, using the resist pattern 36 as a mask.

Subsequently, as shown in FIG. 5B, the resist pattern 36 is removed, andthermal treatment, for example, at a temperature of about 1100° C. forabout 30 minutes is performed to activate the implanted boron ion. Then,the protection oxide film 35 is removed, and a P⁻-type anode epitaxiallayer 2 of about 10 μm in thickness is formed by CVD on the principalsurface of the semiconductor substrate 1 in which the P⁺-type anodeburied layer 1 a is formed.

Next, as shown in FIG. 5C, a high concentration P⁺-type isolation layer14 functioning as a part of an anode is formed selectively in theperipheral portion of the photodetector section 220 in the upper part ofthe P⁻-type epitaxial layer 2 by ion implantation. Then, a N-typeepitaxial layer 3 in which phosphorous (P) ion as a N-type impurity isintroduced at a concentration of about 1×10¹⁶ cm⁻³ is grown by CVD tohave a thickness of, for example, about 1.0 μm.

Subsequently, as shown in FIG. 5D, an isolation oxide film 13 is formedselectively in the N-type epitaxial layer 3 in the photodetector section220 and another isolation oxide film 13 is formed entirely in the N-typeepitaxial layer 3 in the light emitting element section 240. Then, aphotodiode is formed in the photodetector section 220, likewise thefirst embodiment.

Next, as shown in FIG. 5E, anisotropic wet etching using an alkalineetching solution containing KOH or the like is performed on the P⁻-typeepitaxial layer 2 and the upper part of the semiconductor substrate 1,using the isolation oxide film 13 remaining in the light emittingelement section 240 as a mask for trench formation, to form a micromirror region 23 of a trench in the light emitting element section 240.

Subsequently, as shown in FIG. 5F, a laser wire 27 extending from thebottom face of the trench to the upper face of the N-type epitaxiallayer 3 and a laser lower electrode 28 on the laser wire 27 on thebottom face of the trench are formed selectively.

Next, as shown in FIG. 5G, a semiconductor laser chip 25 is bonded onthe laser lower electrode 28, thereby obtaining the opticalsemiconductor device according to the second embodiment.

SECOND FABRICATION METHOD IN SECOND EMBODIMENT

A second fabrication method of the optical semiconductor deviceaccording to the second embodiment will be described below withreference to FIG. 6A through FIG. 6E. In the second fabrication method,a P⁺-type anode buried layer 1 a and a N-type epitaxial layer 3 areformed after a P⁻-type epitaxial layer 2 is formed on the principalsurface of a semiconductor substrate 1.

First, as shown in FIG. 6A, a P⁻-type epitaxial layer 2 is grown on theprincipal surface of the semiconductor substrate 1 by CVD to have athickness of about 10 μm.

Next, as shown in FIG. 6B, a protection oxide film 35 made of siliconoxide is formed on the P⁻-type epitaxial layer 2, and a resist pattern36 as a mask for covering the protection oxide film 35 in a lightemitting element section 240 is formed by a lithography method. Then, aP⁺-type anode buried layer 1 a is formed selectively in a region betweenthe semiconductor substrate 1 and the P⁻-type epitaxial layer 2 exceptthe light emitting element section 240 by ion implantation of, forexample, boron (B) ion as a P-type impurity to the semiconductorsubstrate 1 through the protection oxide film 35 and the P⁻-typeepitaxial layer 2, using the resist pattern 36 as a mask. Herein, thedose amount of the boron ion is set to be 5×10¹⁴ cm⁻² and theacceleration energy is set to be 2 MeV, for example. Thereafter, theresist pattern 36 is removed by ashing or the like, and thermaltreatment, for example, at a temperature of about 1100° C. for about 30minutes is performed on the semiconductor substrate 1 in which the boronion is implanted to activate the boron ions.

Subsequently, as shown in FIG. 6C, a high concentration P⁺-typeisolation layer 14 functioning as a part of an anode is formedselectively by ion implantation in the upper part of the P⁻-typeepitaxial layer 2 in the peripheral portion of the photodetector section220. Then, a N-type epitaxial layer 3 in which phosphorous (P) ion as aN-type impurity is introduced at a concentration of about 1×10¹⁶ cm⁻³ isformed by CVD to have a thickness of about 1.0 μm.

Next, as shown in FIG. 6D, an isolation oxide film 13 is selectivelyformed in the upper part of the N-type epitaxial layer 3 in thephotodetector section 220 so as to cover a part of P⁺-type isolationlayer 14. Simultaneously, another isolation oxide film 13 is formedentirely in the upper part of the N-type epitaxial layer 3 in the lightemitting element section 240. Then, a photodiode is formed in thephotodetector section 220, likewise the first embodiment.

Subsequently, as shown in FIG. 6E, anisotropic etching is performed onthe P⁻-type epitaxial layer 2 and the upper part of the semiconductorsubstrate 1 in the light emitting element section 240, using theisolation oxide film 13 remaining in the light emitting element section240, to form a trench in the P⁻-type epitaxial layer 2 and the upperpart of the semiconductor substrate 1, thereby obtaining a micro mirrorregion 23. Thereafter, a semiconductor laser chip 25 is bonded on thebottom face of the trench, likewise the first fabrication method, toobtain the optical semiconductor device according to the secondembodiment.

THIRD EMBODIMENT

The third embodiment of the present invention will be described belowwith reference to the drawings.

FIG. 7 shows a schematic construction in section of an opticalsemiconductor device (OEIC device) according to third embodiment of thepresent invention. In FIG. 7, the same reference numerals are assignedto the same members as those in FIG. 4 and the description thereof isomitted. In the optical semiconductor device according to the thirdembodiment, a transistor section 200 composed of a NPN bipolartransistor is formed in the epitaxial substrate composed of the P⁻-typeepitaxial layer 2 and the N-type epitaxial layer 3, which composes theoptical semiconductor device according to the second embodiment, inaddition to the photodetector section 220 and the light emitting elementsection 240.

The addition of the transistor section 200 allows an output signal fromthe photodiode of the photodetector section 220 to be input to the NPNtransistor and further to an electronic circuit composed of a resistanceelement and a capacitance element (not shown). Then, the thus inputsingal is amplified and is signal-processed by the electronic circuit,and then, is output as a recording or replay signal of an optical disk.

The transistor section 200 is a tow-layer polysilicon self aligned typeNPN bipolar transistor and is formed outside the isolation oxide film 13located opposite the light emitting element section 240 with respect tothe photodetector section 220 in the P⁻-type epitaxial layer 2 and theN-type epitaxial layer 3.

Referring to the detailed construction of the transistor section 200, itincludes: a high concentration N-type emitter region 6 formedselectively in the upper part of the N-type epitaxial layer 3 by solidphase diffusion; a P-type base region 7 formed under the emitter region6; a collector region 8 made of the N-type epitaxial layer 3 and formedbelow the base region 7; a high concentration N-type collector buriedregion 9 formed below the collector region 8; an emitter electrode 10formed on the emitter region 6; a base electrode 11 connectedelectrically to the peripheral portion of the base region 7; and acollector electrode 12 formed above the collector buried region 9 andconnected electrically to the end part of the collector buried region 9.Wherein, the two-layer polysilicon self aligned type means, for example,a structure in which: a P-type polysilicon is intervened between theP-type base region 7 and the base electrode 11 to solid-phase diffuse aP-type impurity from the polysilicon layer to the upper part of theN-type epitaxial layer 3, thereby forming a P-type base contact layer inthe upper part of the N-type epitaxial layer 3; while a N-typepolysilicon layer is intervened between the N-type collector buriedlayer 9 and the collector electrode 12 to solid-phase diffuse a N-typeimpurity from the polysilicon layer to the upper part of the N-typeepitaxial layer 3, thereby forming a N-type collector contact layer inthe upper part of the N-type epitaxial layer 3.

As described above, in the third embodiment, the transistor section 200and the photodetector section 220 are formed on a single semiconductorsubstrate 1, and therefore, the wiring distance between thephotodetector section 220 and an electronic circuit becomes shorter thanthose in the first and second embodiments. As a result, parasiticcapacitance and inductance can be reduced, which enhances the frequencycharacteristic in the optical semiconductor device and is advantageousin high-speed operation. Further, the transistor section 200, thephotodetector section 220 and the light emitting element section 240 canbe integrated in a single substrate, thereby enabling size reduction ofthe optical semiconductor device.

In addition, similar to the first embodiment, the P⁺-type anode buriedlayer 1 a is not provided in the light emitting element section 240, andaccordingly, no difference in impurity concentration is caused betweenthe semiconductor substrate 1 and the P⁻-type epitaxial layer 2, both ofwhich have low concentration. In this connection, no difference in etchrate, which is caused due to difference in impurity concentration, iscaused in the process of forming the micro mirror region 23 byanisotropic etching using an alkaline solution, thereby preventing aline and a pit, which is developed due to crystal defect from animpurity in the high concentration P⁺-type anode buried layer 1 a, inthe mirror face. Thus, the micro mirror region 23, which is excellent inflatness, can be formed.

FIRST FABRICATION METHOD IN THIRD EMBODIMENT

A first fabrication method of the optical semiconductor deviceconstructed as above will be described below with reference to thedrawings. FIG. 8A through FIG. 8G are sections showing the sequence ofthe first fabrication method of the optical semiconductor deviceaccording to the third embodiment of the present invention.

First, as shown in FIG. 8A, likewise the first fabrication method in thefirst embodiment, a protection oxide film 35 made of silicon oxide isformed on the principal surface of a semiconductor substrate 1 made ofP⁻-type silicon, and a resist pattern 36 as a mask for covering theprotection oxide film 35 in a light emitting element section 240 isformed. Then, a P⁺-type anode buried layer 1 a is formed selectively ina region of the semiconductor substrate 1 except the light emittingelement section 240 by ion implantation of, for example, boron (B) ionas a P-type impurity to the semiconductor substrate 1 through theprotection oxide film 35, using the resist pattern 36 as a mask.

Subsequently, as shown in FIG. 8B, after the resist pattern 36 and theprotection oxide film 35 are removed, a P⁻-type epitaxial layer 2 isgrown by CVD on the principal surface of the semiconductor substrate 1,in which the P⁺-type anode buried layer 1 a is formed, to have athickness of about 10 μm. Then, thermal treatment, for example, at atemperature of about 1100° C. for about 30 minutes is performed on thesemiconductor substrate 1 on which the P⁻-type epitaxial layer 2 isformed to activate the implanted boron ion.

Next, as shown in FIG. 8C, ion implantation of, for example, arsenic(As) ion as a N-type impurity is performed selectively on a region ofthe P⁻-type epitaxial layer 2 between P⁺-type isolation layer formationportions in the transistor section 200 to form a collector buried region9 in a region between the P⁺-type isolation layer formation portions inthe P⁻-type epitaxial layer 2 in the transistor section 200. Herein, thedose amount of arsenic ion is set to be 1×10¹⁶ cm⁻² and the accelerationenergy is set to be 40 keV. Then, high concentration P⁺-type isolationlayers 14 functioning as a part of an anode are formed selectively inthe P⁻-type epitaxial layer 2 in the peripheral portion of thephotodetector 220 by ion implantation. Wherein, one of the P⁺-typeisolation layers 14 which is located in the boundary portion between thetransistor section 200 and the photodetector section 220 is used incommon to the transistor section 200 and the photodetector section 220.Thereafter, a N-type epitaxial layer 3 in which, for example,phosphorous (P) ion as a N-type impurity is implanted at a concentrationof 1×10¹⁶ cm⁻³ is grown by CVD to have a thickness of, for example,about 1.0 μm.

Subsequently, as shown in FIG. 8D, an isolation oxide film 13 is formedselectively in the N-type epitaxial layer 3 in the transistor section200 and the light emitting element section 220 and another isolationoxide film 13 is formed entirely in the N-type epitaxial layer 3 in thelight emitting element section 240. Then, a NPN transistor is formed inthe transistor section 200 and a photodiode is formed in thephotodetector section 220. In the NPN transistor, for example, a P-typebase region 7 is formed selectively in the upper part of the N-typeepitaxial layer 3, and then, a N-type emitter region 6 is formedselectively in a part of the upper part of the base region 7 in theN-type epitaxial layer 3. Thereafter, an emitter electrode 10, a baseelectrode 11 and a collector electrode 12 are formed sequentially.Further, a photodiode in the photodetector section 220 is formed,likewise the first embodiment.

Next, as shown in FIG. 8E, anisotropic wet etching using an alkalineetching solution containing KOH or the like is performed on the P⁻-typeepitaxial layer 2 and the upper part of the semiconductor substrate 1,using the isolation oxide film 13 remaining in the light emittingelement section 240 as a mask for trench formation, to form a micromirror region 23 of a trench in the light emitting element section 240.

Subsequently, as shown in FIG. 8F, a laser wire 27 extending from thebottom face of the trench to the upper face of the N-type epitaxiallayer 3, and a laser lower electrode 28 on the laser wire 27 on thebottom face of the trench are formed selectively.

Next, as shown in FIG. 8G, a semiconductor laser chip 25 is bonded onthe laser lower electrode 28, thereby obtaining the opticalsemiconductor device according to the third embodiment.

SECOND FABRICATION METHOD IN THIRD EMBODIMENT

A second fabrication method of the optical semiconductor deviceaccording to the third embodiment will be described below with referenceto FIG. 9A through FIG. 9E. In the second fabrication method, a P⁺-typeanode buried layer 1 a and a N-type epitaxial layer 3 are formed after aP⁻-type epitaxial layer 2 is formed on the principal surface of asemiconductor substrate 1.

First, as shown in FIG. 9A, a P⁻-type epitaxial layer 2 is grown on theprincipal surface of the semiconductor substrate 1 by CVD to have athickness of about 10 μm.

Next, as shown in FIG. 9B, a protection oxide film 35 made of oxidesilicon is formed on the P⁻-type epitaxial layer 2, and a resist pattern36 as a mask for covering the protection oxide film 35 in a lightemitting element section 240 is formed by a lithography method. Then, aP⁺-type anode buried layer 1 a is formed selectively in a region betweenthe semiconductor substrate 1 and the P⁻-type epitaxial layer 2 exceptthe light emitting element section 240 by ion implantation of, forexample, boron (B) ion as a P-type impurity to the semiconductorsubstrate 1 through the protection oxide film 35 and the P⁻-typeepitaxial layer 2, using the resist pattern 36 as a mask. Herein, thedose amount of the boron ion is set to be 5×10¹⁴ cm⁻² and theacceleration energy is set to be 2 MeV, for example. Thereafter, theresist pattern 36 is removed by ashing or the like, and thermaltreatment, for example, at a temperature of about 1100° C. for about 30minutes is performed on the semiconductor substrate 1 in which the boronion is implanted to activate the boron ion.

Subsequently, as shown in FIG. 9C, a collector buried region 9 is formedin a region of the transistor section 200 between P⁺-type isolationlayer formation portions in the P⁻-type epitaxial layer 2 by selectiveion implantation of, for example, arsenic (As) ion as a N-type impurityto a region of the transistor section 200 between the P⁺-type isolationlayer formation portions in the P⁻-type epitaxial layer 2. Herein, thedose amount of the arsenic ion is set to be 1×10¹⁶ cm⁻² and theacceleration energy is set to be 40 MeV, for example. Then, highconcentration P⁺-type isolation layers 14 functioning as a part of ananode are formed by ion implantation selectively in the upper part ofthe P⁻-type epitaxial layer 2 in the peripheral portion of thephotodetector section 220. Wherein, one of the P⁺-type isolation layers14 which is located in the boundary portion of the transistor section200 and the photodetector section 220 is used in common to thetransistor section 200 and the photodetector section 220. Then, afterthe protection oxide film 35 is removed, a N-type epitaxial layer 3 inwhich phosphorous (P) ion as a N-type impurity is introduced at aconcentration of about 1×10¹⁶ cm⁻³ is grown on the P⁻-type epitaxiallayer 2 by CVD to have a thickness of about 1.0 μm.

Next, as shown in FIG. 9D, an isolation oxide film 13 is selectivelyformed in the upper part of the N-type epitaxial layer 3 in thetransistor section 200 and the photodetector section 220 and anotherisolation oxide film 13 is formed entirely in the upper part of theN-type epitaxial layer 3 in the light emitting element section 240.Then, a NPN transistor is formed in the transistor section 200 and aphotodiode is formed in the photodetector section 220.

Subsequently, as shown in FIG. 9E, a micro mirror region 23 of a trenchis formed in the light emitting element section 240 by etching using theisolation oxide film 13 remaining in the light emitting element section240 as a mask. Thereafter, a semiconductor laser chip 25 is bonded onthe bottom face of the trench, likewise the first fabrication method, toobtain the optical semiconductor device according to the thirdembodiment.

It should be noted that silicon is used for the semiconductor substrate1 in the first to third embodiments according to the present invention,but the semiconductor substrate 1 is not limited to silicon andsemiconductor substrates made of germanium or compound semiconductorswhich are generally used in optical devices handling a longer wavelengthregion may be used.

Moreover, PIN photodiode is used for the photodetector section 220 ineach embodiment, but the present invention is applicable to an ordinaryPN-type photodiode, an avalanche photodiode and a phototransistor, ofcourse.

As described above, the optical semiconductor devices and thefabrication methods therefor according to the present invention exhibitan effect that a photodetector having high-speed operability and highphotosensitivity and a semiconductor laser element can be formed in asingle substrate, with no optical characteristic deteriorated, and isuseful in optical semiconductor devices in which a photodetector and asemiconductor laser element are mounted and the fabrication methodsthereof.

1. An optical semiconductor device comprising: a first conductivity typefirst semiconductor region; a first conductivity type secondsemiconductor region formed on the first semiconductor region; a secondconductivity type third semiconductor region formed on the secondsemiconductor region; a photodetector section formed of the secondsemiconductor region and the third semiconductor region; a micro mirrorformed of a trench formed selectively in a region of the firstsemiconductor region and the second semiconductor region except thephotodetector section; and a semiconductor laser element held on abottom face of the trench, wherein a first conductivity buried layer ofwhich impurity concentration is higher than those of the firstsemiconductor region and the second semiconductor region is formedbetween the first semiconductor region and the second semiconductorregion in the photodetector section.
 2. The optical semiconductor deviceof claim 1, wherein the second semiconductor region is formed byepitaxial growth.
 3. The optical semiconductor device of claim 2,wherein the third semiconductor region is formed by epitaxial growth. 4.The optical semiconductor device of claim 1, further comprising: atransistor formed in a region of the second semiconductor region and thethird semiconductor region except the photodetector section and thetrench.
 5. An optical semiconductor device fabrication method,comprising the steps of: forming selectively a first conductivity typeburied layer of which impurity concentration is higher than that of afirst conductivity type first semiconductor region by ion implantationto a photodetector section formation portion of the first semiconductorregion; forming by epitaxial growth a first conductivity type secondsemiconductor region of which impurity concentration is lower than thatof the buried layer on the first semiconductor region in which theburied layer is formed; forming a second conductivity type thirdsemiconductor region in the upper part of the second semiconductorregion; forming a photodetector section made of the second semiconductorregion and the third semiconductor region in the photodetector sectionformation portion of the second semiconductor region and the thirdsemiconductor region; forming, by forming a trench by performingselective anisotropic etching on a region of the first semiconductorregion and the second semiconductor region except the photodetectorsection, a micro mirror formed of a wall face of the trench; and bondinga semiconductor laser element, which is prepared beforehand in a form ofa chip, onto a bottom face of the thus formed trench.
 6. The opticalsemiconductor device fabrication method of claim 5, further comprisingthe step of: forming selectively a transistor in a region of the secondsemiconductor region and the third semiconductor region except thephotodetector section and the trench.
 7. An optical semiconductor devicefabrication method comprising the steps of: forming selectively a firstconductivity type buried layer of which impurity concentration is higherthan that of a first conductivity type first semiconductor region by ionimplantation to a photodetector section formation portion of the firstsemiconductor region; forming by epitaxial growth a first conductivitytype second semiconductor region of which impurity concentration islower than that of the buried layer on the first semiconductor region inwhich the buried layer is formed; forming a second conductivity typethird semiconductor region on the second semiconductor region byepitaxial growth; forming a photodetector section made of the secondsemiconductor region and the third semiconductor region in thephotodetector section formation portion of the second semiconductorregion and the third semiconductor region; forming, by forming a trenchby performing selective anisotropic etching on a region of the firstsemiconductor region and the second semiconductor region except thephotodetector section, a micro mirror formed of a wall face of thetrench; and bonding a semiconductor laser element, which is preparedbeforehand in a form of a chip, onto a bottom face of the thus formedtrench.
 8. The optical semiconductor device fabrication method of claim7, further comprising the step of: forming selectively a transistor in aregion of the second semiconductor region and the third semiconductorregion except the photodetector section and the trench.
 9. An opticalsemiconductor device fabrication method comprising the steps of: forminga first conductivity type second semiconductor region on a firstconductivity type first semiconductor region by epitaxial growth;forming selectively a first conductivity type buried layer of whichimpurity concentration is higher than that of the first semiconductorregion by ion implantation to a boundary portion between the firstsemiconductor region and the second semiconductor region and aphotodetector section formation portion in the vicinity of the boundaryportion; forming a second conductivity type third semiconductor regionin an upper part of the second semiconductor region; forming aphotodetector section made of the second semiconductor region and thethird semiconductor region in a photodetector section formation portionof the second semiconductor region and the third semiconductor region;forming, by forming a trench by performing selective anisotropic etchingon a region of the first semiconductor region and the secondsemiconductor region except the photodetector section, a micro mirrorformed of a wall face of the trench; and bonding a semiconductor laserelement, which is prepared beforehand in a form of a chip, onto a bottomface of the thus formed trench.
 10. The optical semiconductor devicefabrication method of claim 9, further comprising the step of: formingselectively a transistor in a region of the second semiconductor regionand the third semiconductor region except the photodetector section andthe trench.
 11. An optical semiconductor device fabrication methodcomprising the steps of: forming a first conductivity type secondsemiconductor region on a first conductivity type first semiconductorregion by epitaxial growth; forming selectively a first conductivitytype buried layer of which impurity concentration is higher than that ofthe first semiconductor region by ion implantation to a boundary portionbetween the first semiconductor region and the second semiconductorregion and a photodetector section formation portion in the vicinity ofthe boundary portion; forming a second conductivity type thirdsemiconductor region on the second semiconductor region by epitaxialgrowth; forming a photodetector section made of the second semiconductorregion and the third semiconductor region in a photodetector sectionformation portion of the second semiconductor region and the thirdsemiconductor region; forming, by forming a trench by performingselective anisotropic etching on a region of the first semiconductorregion and the second semiconductor region except the photodetectorsection, a micro mirror formed of a wall face of the trench; and bondinga semiconductor laser element, which is prepared beforehand in a form ofa chip, onto a bottom face of the thus formed trench.
 12. The opticalsemiconductor device fabrication method of claim 11, further comprisingthe step of: forming selectively a transistor in a region of the secondsemiconductor region and the third semiconductor region except thephotodetector section and the trench.